A fast breeder reactor using liquid sodium as a coolant has a core comprising a core region charged with a fuel material prepared by enriching plutonium, a fissile material, and an external blanket region surrounding said core region and moreover charged with a fuel material (e.g. natural uranium or depleted uranium) whose main component is a fertile material (e.g. uranium-238). The output of the fast breeder reactor is controlled by moving control rods in and out of the core. Each control rod is provided with a plurality of neutron absorber rods packed inside with B.sub.4 C, a neutron absorber. The length of a region packed with the neutron absorber inside a neutron absorber rod is equal to that of the aforesaid core region. The control rods are moved vertically through control rod guide tubes installed in the core. The control rods are classified roughly into two kinds according to their functions. One is an output regulation control rod; the other is a reactor shutdown control rod. The output regulation control rod is inserted into the core region in the initial stage of operation of the fast breeder reactor to limit initial burnup reactivity. Then, it raised from the core region as burnup reactivity lowers as the operation proceed, and is raised completely out of the core region in the last stage of the operation. The output regulation control rod, however, is reinserted into the core region when the operation of the fast breeder reactor is stopped. Meanwhile, the reactor shutdown control rods are all held up outside the core region when the reactor is started, and are not inserted into the core region during the operation of the reactor. They are inserted thereinto when the operation of the reactor is stopped. In other words, the output regulation control rod regulates the output of the fast breeder reactor and stops the reactor, while the reactor shutdown control rod shuts down the reactor. The number of output regulation and reactor shutdown control rods is about equal.
The neutron absorber rod in a control rod as disclosed in the Official Gazette on Japanese Patent Laid-Open No. 65794/1975 is provided, in the upper inside, with a gas plenum which accumulates He gas produced through the absorption of neutrons by B.sub.4 C. This gas plenum, however, does not increase core reactivity.